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Hi,

in the past I had tried to implement the RS-232 interface to my Challenger 1P by following the instructions in "The first book of Ohio Scientifc Vol. I" and in the Professor Mark Csele: Ohio Scientific Computers web page. Unfortunately I never was able to make it work, and I abandoned the project. I then fell back to loading WAV files for BASIC programs and cc65-compiled programs over the headphone port of my laptop.

The implementation of the RS-232 interface required to scratch out a pcb track on the bottom of the C1P board, and the addition of switch to toggle between RS-232 and cassette operation. At some point in time I decided to remove the RS-232 connector and the switch, as it was useless anyway. When I yesterday tried again to load a program over the cassette port, it didn't work, and I was completely stumped first, because previously it had worked very reliably. Only after re-reading the instructions for the RS-232 implementation in the book and in the corresponding section in the Professor Mark Csele: Ohio Scientific Computers web page it occurred to me that I had to add back a jumper to make the cassette port work again.

And here I got confused. On Mark Csele's web page it says this:

The actual 'W10' jumper which must be cut is located on the bottom side of the board and detailed in the photograph. With this jumper removed, even cassette operation requires a jumper on J3.

Connector J3 Wiring (for Cassette Tape operation):

Jumper J3 Pin 5 to J3 Pin 6

Connector J3 Wiring (for RS-232 operation):

Jumper J3 Pin 4 to J3 Pin 6
J3 Pin 2 (RS-232 Out) to DB-9 Pin 2 (Transmit to PC)
J3 Pin 3 (RS-232 In) to DB-9 Pin 3 (Receive from PC)
Board Ground to DB-9 Pin 5 (Ground)

However in "The first book of Ohio Scientifc Vol. I" on page 80 it says this about installing the switch:

Code: Select all

CONNECTOR AND SWITCH FABRICATION

The first six pins on the molex connector, part #P2, are to be connected as follows:

PIN #       To location
...
4             Pole of SPDT switch, part # S1 ...
5             Out 1 of SPDT switch, part # S1
6             Out 2 of SPDT switch, part # S1

Note that Mark Csele's instructions tell to connect pin 6 to either pin 5 or pin 4 for toggling between cassette and RS-232, while the installation of the switch according to the book will connect pin 4 to either pin 5 or pin 6 for toggling.

My own experiments seem to confirm that connecting pin 4 to pin 5 is the correct thing to do to enable cassette operation.

Can someone confirm or rebut that the instructions in the book are correct? I the book is correct, would contact Mark then for a correction of his instructions.

Thanks
Stephan

What revision is your board I'm pretty sure it makes a difference as to how it gets wired

Klyball wrote:What revision is your board I'm pretty sure it makes a difference as to how it gets wired

It says "Ohio Scientific Model 600 CPU 1978 REV B" on the board.

did you install the transistors or were they already in there, check that there in there correctly

Klyball wrote:did you install the transistors or were they already in there, check that there in there correctly

I have installed the two transistors for the RS-232 interface myself, and yes, I had checked that they are installed correctly.

Stephan,

How were you connecting the OSI to your host computer? This technique for RS-232 interfacing doesn't exactly use the correct voltage levels and may not work with all interfaces.

Do you have an oscilloscope? That could be a huge help.

BTW, When I retrieved my old OSI back around 2000, I just wired in a MAX-232A chip, directly to the ACIA Tx and Rx pins, and to a serial port on the back. That works with everything, although not period correct. Also, you can get USB-to-TTL converters that you can wire in to the ACIA. They will work perfectly as well.

Dave

dave wrote:Stephan,

How were you connecting the OSI to your host computer? This technique for RS-232 interfacing doesn't exactly use the correct voltage levels and may not work with all interfaces.

I was using an USB to RS-232 converter.

dave wrote:Do you have an oscilloscope? That could be a huge help.

No, I don't have an oscilloscope. What kind of oscilloscope would I need?

dave wrote:BTW, When I retrieved my old OSI back around 2000, I just wired in a MAX-232A chip, directly to the ACIA Tx and Rx pins, and to a serial port on the back. That works with everything, although not period correct. Also, you can get USB-to-TTL converters that you can wire in to the ACIA. They will work perfectly as well.

Dave

I have this USB to TTL Serial Cable. Should that work? How would I connect it to the OSI C1P?

Thanks
Stephan

Mark's note appears to be incorrect.
J3-4 (normally the pole of CASS/RS-232 switch) connects the ACIA receive input (PIN2) to J3-5 for CASS or J3-6 for RS232. As per FBOSI article which is correct.
Yes, if your not using the switch after breaking W10 you link J3 4-5 for CASS or J3 4-6 for RS-232
Best option- restore the switch,fix your RS232 by any method and do 600baud min mod.
Mike
PS Stephan has a very tidy original 79 C1 revB

Without a scope you can still verify RS-232 operation with a multimeter.

*=$1600

A9 00 ;/FF 00 gives approx 3V at J3-2, FF gives 0.4V
48
AD 00 F0
4A
4A
90 F9
68
8D 01 F0
4C 00 16

You can also check bits are toggling if your meter shows frequencies as well.Changing $1601 from AA to CC will halve the frequency measured. Use a loopback plug (or loop J3 2-3 ) to check the RX side of things.

MK14HAK wrote:Mark's note appears to be incorrect.

J3-4 (normally the pole of CASS/RS-232 switch) connects the ACIA receive input (PIN2) to J3-5 for CASS or J3-6 for RS232. As per FBOSI article which is correct.
Yes, if your not using the switch after breaking W10 you link J3 4-5 for CASS or J3 4-6 for RS-232[/quote]

Ok, I will try to contact Mark to discuss the information on his web page.

MK14HAK wrote:Best option- restore the switch,fix your RS232 by any method and do 600baud min mod.
Mike
PS Stephan has a very tidy original 79 C1 revB

Without a scope you can still verify RS-232 operation with a multimeter.

*=$1600

A9 00 ;/FF 00 gives approx 3V at J3-2, FF gives 0.4V
48
AD 00 F0
4A
4A
90 F9
68
8D 01 F0
4C 00 16

Thank you for this little test program! Indeed I measure approx 3.5V with 00 in address 1601, and 0.4V with FF in address 1601. I'm starting to believe that I made a mistake in the wiring of the 9-PIN serial connector that I used to attach the USB serial interface of my laptop to.

MK14HAK wrote:You can also check bits are toggling if your meter shows frequencies as well.Changing $1601 from AA to CC will halve the frequency measured. Use a loopback plug (or loop J3 2-3 ) to check the RX side of things.

How can I check the RX side with the loop J3 2->3? Would I do this from BASIC by writing something out, and then it should be echoed automatically?

stm wrote:How can I check the RX side with the loop J3 2->3? Would I do this from BASIC by writing something out, and then it should be echoed automatically?

Nevermind, I actually had wired the 9-pin female RS-232 connector in a wrong way. After taking another look at the pin layout and after resoldering it, it now works! I was able to successfully transfer data between the C1P and my laptop.

Klyball, Mike and Dave, thanks for bringing me on the right track! After I initially had started this thread during my attempts to permanently discard the modifications for the non-functional serial interface, I now have a working RS-232 connection